As optical fiber communication channels increasingly replace metal cable and microwave transmission links, optical waveguiding apparatus in the form of integrated optical waveguide devices become increasingly important. Such devices typically comprise a substrate, such as silicon, provided with a cladding base layer such as SiO2, a thin patterned core layer over the base, and a top cladding layer over the patterned core. The core has a higher refractive index than the cladding layers to provide waveguiding properties, and the core layer is configured, as by photolithographic techniques, to perform any one of a wide variety of optical processing functions such as beam splitting, tapping, multiplexing, demultiplexing and filtering.
With the advent of higher transmission rates and increasing levels of wavelength division multiplexing, it has become desirable to provide waveguiding apparatus with an increasing density of processing devices operating on an increasing number of optical inputs. Compact design of such apparatus requires waveguide “crossovers” where one guided beam crosses another. Typically the waveguiding core regions do not physically cross over on different planes, but rather pass through the same coplanar region.
A difficulty with waveguide crossovers is that they engender optical loss through scattering and cross talk as some light from each path goes to the other. The intersecting waveguides present an asymmetric index profile at the crossing. This profile disturbs the guided optical mode and excites higher order optical modes. Since the intersection region is abrupt (non-adiabatic), it will excite non-guided modes, resulting in crosstalk and the loss of optical power. These problems are aggravated as the waveguide index contrast δ increases.
FIG. 1A, which is prior art, schematically illustrates a conventional crossover comprising a pair of core optical waveguides 10 and 11 intersecting at an angle Ø in a common core layer over a region 12. The core waveguides 10, 11 have an index of refraction n2 higher than the index n1 of the surrounding cladding layers. For high density apparatus, the crossovers typically intersect at a small angles Ø<5° and have cores presenting a high index contrast to the cladding, i.e. a high value of delta=(n2−n1)/n2. But even with low index contrast, propagating optical modes are disturbed by the intersection region, exciting non-guided modes that cause loss of optical power and crosstalk.
FIG. 1B is a graphical illustration showing optical power loss as a function of the crossing angle Ø for typical conventional crossovers having delta=4% (curve 1) and delta=0.8% (curve 2). It is clear that loss increases rapidly as the crossing angle decreases.
Many techniques have been proposed for reducing losses at the waveguide crossing. One approach is to up-taper the guiding layer to increase its width as the waveguides approach the intersecting region. (See K. Aretz et al., “Reduction of crosstalk and losses of intersecting waveguide,” 25 Electronics Letters, No. 11 (May 25, 1989); see also H. G. Bukkens, et al., “Minimization of the Loss of Intersecting Waveguides in InP-Based Photonic Integrated Circuits,” IEEE Photonics Technology Letters, No. 11 (November 1999)). The optical beam size, therefore, expands at the crossing, which makes a better matching of the optical mode to the waveguide at the other side of the crossing. Low crosstalk of>30 dB and low loss were achieved for angles>6°. However, this technique requires very long tapering length (>1 mm), which is impractical for certain applications. Also, the technique is not effective for high delta waveguide crossings.
A similar approach is proposed by Hernandez et al. in U.S. Pat. No. 4,961,619 issued Oct. 9, 1990. The width of the waveguide is increased or decreased at the crossing junction to modify the optical mode characteristics in that region. This introduces an axial variation in the transverse index of refraction distribution, which allows for better alignment of the electrical field at the crossing. The method can also be used for small angle crossing below 5°. However, it is not very suitable for high delta waveguide since it requires large tapering regions to adiabatically expand the optical mode.
In a third approach by Nishimoto (U.S. Pat. No. 5,157,756 issued Oct. 20, 1992), the index of the intersecting region includes a peripheral region of low index surrounding an island of waveguide material at the center of the crossing. See also Lemoff, et al. This technique could reduce the losses for small angle crossing. However, it is not effective for high step index contrast waveguides, and the loss is expected to be higher.
Accordingly, there is a need for optical waveguiding apparatus having waveguide crossovers of reduced loss.